1. Field of the Invention
The present invention relates to an impact tool such as an electric hammer which is provided with a striker operable to deliver impact to a tool bit through pneumatic spring action.
2. Description of the Prior Art
A conventional impact tool such as an electric hammer includes a body having a barrel for receiving a movable cylinder which is reciprocally movable in the barrel. The cylinder in turn receives a striker which is reciprocally movable therewithin. An air chamber is formed within the cylinder defined by the striker. The air within the air chamber is compressed by the reciprocal movement of the cylinder and the striker is moved toward the tool bit to deliver impact thereto by the compressed air force or pneumatic spring action.
To prevent idling or idle percussive action of the striker, the cylinder includes an air replenishing port and an air escape port at predetermined positions. The air chamber communicates with the outside through the air escape port when the striker is positioned at a position in the vicinity of its forward stroke end in the case that the tool bit is not mounted on the barrel or the tool bit is not pressed on a work. The air within the air chamber is, therefore, not compressed by the reciprocal movement of the cylinder, so that the idle percussive action of the striker is prevented. The impact operation can be made by the reciprocal movement of the cylinder when the tool bit is mounted on the barrel and is pressed on the work so as to move the striker theretogether to a position where the air escape port is closed by the striker.
U.S. Pat. Nos. 3,688,846 and 4,290,492 disclose such conventional impact tools.
In such prior art impact tools, only one air escape port is provided for communication between the air chamber and the outside so as to prevent idling of the striker throughout the reciprocal movement of the cylinder and to permit normal impact operation when it is closed by the striker as stated above. The position of the air escape port in the cylinder is therefore determined in accordance with the stroke of the striker. Thus, when it is desired to construct the cylinder to have a long stroke, the stroke of the tool bit for closing the air escape port as well as that of the striker becomes also longer. Consequently, the length of the whole impact tool becomes longer, so that the weight of the impact tool increases to cause difficulty in handling of the tool.
Further, in the prior art impact tools, the tool bit is pressed on the work so as to move the striker theretogether to a position to close the air escape port so as to obtain air tight of the air chamber. The tool bit is pressed on the work with the whole tool held by the operator, and therefore, the positive pressure by the compressed air in the air chamber creates reaction force against the tool bit, so that the whole tool is lifted. This may prevent reliable operation of the tool including the operation for pressing the tool bit on the work.